Optical Coherence Tomography (OCT) is a form of optical coherence analysis that is typically used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are sent through an object or sample and a computer produces images of cross sections of the object by using information on how the waves are changed.
A common type of optical coherence analysis is termed Fourier domain OCT (FD-OCT). These Fourier domain techniques often use a wavelength swept source and a single detector, sometimes referred to as time-encoded FD-OCT (TEFD-OCT) or swept source OCT (SS-OCT) since it has advantages in speed and signal-to-noise ratio (SNR). The spectral components are encoded in time. The spectrum is either filtered or generated in successive frequency steps and reconstructed before Fourier-transformation.
In TEFD-OCT, critical performance characteristics of the swept source laser are its tuning speed and accuracy. In order to compensate for instabilities and/or non-linearities in the tuning of the wavelength tuned laser, a sampling clock (k-clock) is employed to enable sampling at equally spaced increments in the optical frequency domain (k-space). This k-clock must usually be delayed to match the delay associated with the optical signals in the sample and reference arms of the interferometer of the OCT system.
If a k-clock is not used but the laser tunes non-linearly, other corrective options are employed. Some resample the data equally in k-space by interpolation, see S. Yun, G. Tearney, B. Bouma, B. Park, and J. de Boer, “High-speed spectral-domain optical coherence tomography at 1.3 μm wavelength,” Opt. Express 11, 3598-3604 (2003), or employ a nonuniform discrete Fourier Transform (NDTF) that allows k to vary from an integral value, see S. Sharif, C. Flueraru, Y. M Mao, and S. Chang, Swept source Optical Coherence tomography with Nonuniform Frequency Domain Sampling,” OSA/Biomed/DH/LACSEA, BMD86.pdf (2008).
Resampling in k-space has disadvantages. Typically, to accurately resample, oversampling must be employed, which adds overhead to the signal collection and processing. The Sharif-NDTF solution, since it does not involve a k-clock, relies on scan-to-scan stability in the swept source.
When a k-clock is used, transform-limited reconstruction of swept source OCT images at high speed requires that the frequency clock be well time-synchronized with the interference signal. FIG. 1 shows the computed point-spread function (FFT index) vs. clock delay. This plot shows the effect of clock and signal timing mismatch for a laser sweeping at 10 kHz. The required timing accuracy scales linearly with increases in sweep rate. The point spread function (PSF) is plotted as the clock delay is varied. This is a numerical experiment where real clock and interferometer signals were digitized with high time resolution. The PSF was reconstructed using this data with the clock mathematically advanced or retarded relative to the signal with a nominal delay of about 50 nanoseconds (ns) in the OCT interferometer. A delay 10 nanosecond causes a measureable difference the PSF; such delay corresponds to 2 meters of fiber. This effect would not exist if the laser could be swept linearly—that is at constant change in optical frequency per unit time. Practical limitations of the laser's tuning mechanism often prevent doing this to high accuracy. Generally, this problem becomes more severe with higher sweep frequencies.
The most common solution to delay matching the sampling clock to the interferometer delay is to use an optical delay. Simply, the optical signal used for the k-clock is transmitted through a length of optical fiber that has the same delay as the interferometer delay. The use of the optical delay leads to some logistical challenges such as managing the length of optical fiber used for the delay line, however.
In newer designs, the k-clock system is integrated with the swept laser source. An example is disclosed in U.S. patent application Ser. No. 12/396,099, filed on 02-MAR -2009, entitled Optical Coherence Tomography Laser with Integrated Clock, by Flanders, et al., U.S. Pat. Appl. Publ. No. US 2009/0290167 A1, which is incorporated herein by this reference. Here, the delay in the k-clock is provided electronically. This solution has certain advantages in that the electronic delay can be programmable to match changes in the interferometer delay that might be concomitant with the use of different OCT probes, for example.